In recent years, with expansion of the application field of a lithium secondary battery from electronic devices such as cellular phone, personal computer and digital camera to in-car devices, higher enhancement in the performance of a lithium secondary battery, such as increase of the output density and energy density and reduction of the capacity loss, is proceeding. In particular, the in-car application may involve exposure to a harsh environment as compared with the civilian application and therefore, requires high reliability in terms of cycle life and storage performance.
Conventionally, a nonaqueous electrolytic solution obtained by dissolving a lithium salt in an organic solvent is being used for the electrolytic solution of a lithium secondary battery. The decomposition and side reaction of such a nonaqueous electrolytic solution affect the performance of the lithium secondary battery, and with an attempt to enhance the cycle life and storage performance, a technique of mixing various additives in the nonaqueous electrolytic solution has been used.
A difluorophosphate is known to be advantageous as such an additive. For example, it is disclosed in Patent Document 1 that when a nonaqueous electrolytic solution containing, as the additive, at least either one of lithium monofluorophosphate and lithium difluorophosphate is used, a film can be formed on the positive electrode and the negative electrode of a lithium secondary battery and this enables preventing the electrolytic solution from decomposition due to contact of the nonaqueous electrolytic solution with the positive electrode active material and the negative electrode active and realizing suppression of the self-discharge and enhancement of the storage performance.
With respect to lithium difluorophosphate, its production process is disclosed in Non-Patent Documents 1 to 4 and Patent Documents 2 to 7. Non-Patent Document 1 discloses a method of causing ammonium fluoride, acidic sodium fluoride and the like to act on diphosphorus pentoxide to obtain a difluorophosphate. However, this method suffers from by-production of a monofluorophosphate, a phosphate and water in large amounts other than the difluorophosphate, giving rise to a heavy load in the subsequent purification step, and can be hardly said to be an efficient technique.
Non-Patent Document 2 discloses a method of causing P2O3F4 (difluorophosphoric acid anhydride) to act on an oxide or hydroxide such as Li2O and LiOH to obtain a difluorophosphate. However, this method is disadvantageous to industrial production because the difluorophosphoric acid anhydride used therein is very expensive and moreover, a high-purity product is difficulty available.
Non-Patent Document 3 discloses a method of reacting difluorophosphoric acid and lithium chloride to obtain lithium difluorophosphate. However, in this method, a monofluorophosphate is readily produced as an impurity and high-purity lithium difluorophosphate can be hardly obtained.
Non-Patent Document 4 discloses a technique of melting/reacting urea, potassium dihydrogenphosphate and ammonium fluoride to obtain potassium difluorophosphate. However, this method requires disposal of an ammonia gas by-produced in a large amount and allows remaining of a large amount of ammonium fluoride and can be hardly said to be an efficient technique.
Patent Document 2 describes a method of melting/reacting potassium hexafluorophosphate and potassium metaphosphate to obtain potassium difluorophosphate, but there is a fear of contamination from a crucible used for melting and in view of necessity of as a high temperature environment as 700° C., this method is also not a productive technique.
Patent Documents 3 to 5 disclose a method of reacting lithium hexafluorophosphate with a borate, silicon dioxide or a carbonate in a nonaqueous solvent to obtain lithium difluorophosphate.
Patent Document 6 discloses a method of bringing a carbonate and a borate into contact with a gas such as phosphorus pentafluoride to obtain lithium difluorophosphate. However, in order to obtain a difluorophosphate by such a reaction, a long time of, for example, from 40 to 170 hours is required, which is unsuitable for industrial production.
Patent Document 7 discloses a method of reacting a halide except for fluoride with lithium hexafluorophosphate and water in a nonaqueous solvent to obtain lithium difluorophosphate. However, in this method, the lithium difluorophosphate is obtained only as a mixture with lithium hexafluorophosphate but cannot be obtained as a simple substance. Furthermore, the lithium difluorophosphate is obtained only in a state of being dissolved in a solution, and the operation for compositional adjustment of the electrolytic solution is cumbersome, which is disadvantageous to industrial production.
Patent Document 8 discloses a technique of reacting hexafluorophosphate with a compound having an Si—O—Si bond in an organic electrolytic solution to obtain lithium difluorophosphate, but a step of removing a compound which is newly produced in the system and has a boiling point lower than that of the compound having an Si—O—Si bond, is necessary.